Magnetic plug



Dec. 6, 1960 H. H WALKUP EI'AL 2,963,645

' MAGNETIC PLUG Filed March 19, 1957 INVENTORS ff ni'erfifmwp Band 1!. Afaaeg.

i ATTORNEYS United States Patent MAGNETIC PLUG Hunter H. Walkup, Portsmouth, and David M. Moses,

Norfolk, Va., assignors to the United States of America as represented by the Secretary of the Navy Filed Mar. 19, 1957, Ser. No. 647,170 4 Claims. (c1. s2 4--3s) (Granted under Title as, us. Code 1952), see. 266

specting the inner surfaces of ferromagnetic tubular members for defects or flaws.

Briefly, magnetic inspection of a type to which the invention relates involves a magnetic device that magnetizes an inside portion of the ferromagnetic tubular member being inspected. The magnetic lines of force arrange themselves generally smoothly throughout the metal of the tubular member except at the places where the metal has cracks or flaws. At such places the lines distort, and a magnetic powder or similar magnetic particles spread through or blow into the bore ofthe magnetized tubewill outline the distorted parts of the magnetic field along the surface of the tube, forming a pattern that indicates the flaws. The pattern may be visually observed either directly or through some viewing instrument such as, for example, a borescope.

It is an object of the invention to provide a magnetizing device of a type described for detecting internal flaws of atubular member, and especially a thick-walled tube.

It is a further object of the invention to provide a magnetizing device of a type described which is easy to use and which can be easily constructed from readily avialable materials and hence is inexpensive.

It is a further object of the invention to provide a magnetizing device of the type described which can be easily moved from one test section or portion to another of a tubular member so that the tubular member can be quickly scanned for complete inspection, if desired. Consequently long tubular members can be readily inspected although the operating portion of the magnetizing device itself is comparatively small.

It is still another object of the invention to provide a magnetizing device of the type described which most effectively magnetizes the tubular member to be inspected so that more sensitive inspection is possible.

A magnetic device in accordance with the invention comprises an elongated bar having a portion made of a highly magnetically permeable material, such as for example, soft steel, and comprises a plural-turn electric coil for magnetizing the soft steel portion. The coil is made of insulated wire wrapped around the soft steel portion, the wire being of any suitable type capable of carrying large currents. Soft steel discs on the barportion providean efiicient magnetic path for magnetizing the part of the tubular member involved, so that a high flux density is utilized. For fixing the length of the coil, one of the discs is fixed and a third disc, of nonmagnetic material, is located between the two steel discs, providing a length along the steel bar-portion that holds the coil and serves as the core of the coil. By moving the bar-within the test member, concentrated magnetic fields can be produced along any section of the a tubular member for flaw inspections.

Other objects and many of the attendant features and advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following details and descriptions when considered in connection with the accompanying drawing in which the figures are not to scale but in which like reference numerals designate like parts throughout the figures thereof, and wherein:

Fig. 1 is a view, partly in section, of a tubular member containing a magnetizing device in accordance with the invention;

Fig. 2 is an elevational view of the nonmagnetic disc of the device;

Fig. 3 is a longitudinal sectional view of a further modified form of magnetizing device in accordance with the invention; and V Fig. 4 is an elevational view of a part of an end disc of another modified form of magnetizing device embodying the invention.

In the drawing, the member A represents a tubular member which is to be magnetically inspected for internal surface defects. The member A is preferably annular in cross section and may be, for example, a circular tubular shaft for ship propulsion purposes, or any other similar member. For inspecting the inner surfaces of the member A, a magnetizing device is provided which is represented by the reference character B.

In the preferred form of the magnetizing device B shown in Figs. 1 and 2, the device comprises an elongated central bar 10 having a solid straight ferrous end portion 12 which is preferably soft steel of high magnetic permeability, but the entire bar may be made of the same material. The bar 10 shown in Fig. l is circular and, if desired, the end opposite the magnetic portion 12 can be provided with a grip or other handle. Secured to the free end of the soft steel portion 12 of the bar 10 is a plate 14 in the form of a circular disc, the disc being provided with a center hole which tightly fits on the end of the portion 12 although any suitable securing means may be provided to secure the disc to the bar in a fixed position. If desired the securing means may be such as to permit the disc to be readily removed. A form of securing means shown in Fig. 1 comprises a small plate 16 which is screwed to the end of the rod portion 12 and is fastened to the disc 14 by screws 18, with the end of the rod portion and the fastened face of the disc being substantially flush. A second circular disc 20 of nonmagnetic material, such as plywood, is slipped over the bar 10, and is secured to the bar 10 by any suitable means *such as for example by a nonmagnetic bracket 22 fastened to the plywood disc 20 and the rod 10. Finally, a second soft steel plate in the form of a circular disc 24 is slip fitted on the magnetizable portion 12 of the bar 10, and may be placed in any desired position on the bar-portion 10. Preferably the discs 14 and 24 form fairly good fits on the bar-portion so as to provide a good magnetic contact therewith, but it is desirable to have the disc 24 readily adjustable along the length of the bar-portion 12.

In order to provide a magnetizing the magnetic field, the space between the fixed steel disc 14 and the fixed plywood disc 20 receives a coil 26 that is confined by the discs. The coil 26 is made of wrapping turns of a conductor 28 around the part of the bar-porforce that produces I tion 12 between the discs 14 and 20. In accordance with that a suitable magnetizing coil can be formed in an easy and convenient manner by wrapping flexible weldf ing cable around the bar 10 between the fixed discs 14 and 20. A high magnetizing force is obtained by passing 3 a large current through the cable from any suitable source 30. Since the energizing current need be on for only short or intermittent periods of time, the source may be a welding generator.

An indication of the size of a practicable form of the invention may be gathered from the following description of an actual device used to inspect a propeller shaft of 23 inches outside diameter and about 14 inches inside diameter for inside surface cracks. The bar of the testing device comprised a soft steel bar-portion 3 inches in diameter. One end of the barportion had a soft steel disc 1 /2 inches thick by inches in diameter. The positioning plywood disc was also 10% inches in diameter but only inch thick. The third disc, equivalent'to the movable disc 24, was also 1 /2 inches thick but 8% inches in diameter. Fifty turns of 300 ampere flexible welding cable was wrapped around the bar-portion between the equivalent of end disc 14 and positioning disc 20, the two discs being spaced about 12 inches apart, so that the length of the coil equivalent to coil 26 was the same. With 250 DC. amperes as a magnetizing current through the coil, the entire inside surface of the hollow propeller shaft was satisfactorily inspected for defects.

The coil-ends of conductors 28 pass through a small slot 31 in the peripheral portion of the central disc 20, the groove 31 being of a size snugly to receive them. The undersized diameter of the outer disc 24 permits the conductors to readily pass over the disc. The conductors can be taped by tape 32, or otherwise attached to the rod 10, on their way to the power source 30.

For magnetic inspection of the tube A, the magnetizing device B is first positioned at the end of the tube. A magnetic powder is dusted inside the surface of the tube, preferably after the coil 26 is energized. The particles will arrange themselves along a pattern revealing the flaws in the inner surface of the tube A. By moving the magnetizing device step by step, the whole bore of the tube can be scanned in a way well known to the art.

The diameters of the discs 14 and 24 are not critically related to the inside bore of the tube being tested but they should not be so close in size to that of the tube as to practically eliminate leakage flux, nor so far from that of the tube as to short-circuit the flux so that the fiux does not pass along the surface of the tube being tested.

Also, it is preferred that heavy cable and high currents be used. The coil made of this heavy cable is fixed in position by the discs 14 and 2t and the disc 24 is adjustable lengthwise on the bar-portion 12 so as to best adjust the fiux lines to accommodate different lengths of testsections of the tube, or different diameters of test tubes. Furthermore, all of the discs can be removed and replaced by others of different diameters for best testing of a particular hollow propeller shaft which for marine and military purposes must have a high degree of freedom from defects.

Fig. 3 shows a modification wherein the conductor ends of coil 26 pass through a peripheral slot in disc 14' and then inwardly to and through the soft steel portion 12 of bar 10' which, in this case is hollow. The bar 10 of this modification comprises the soft steel portion 12, an end of which fits an inner stepped groove of a wood portion 60 of the bar 10". The coil 26 is held between the soft steel disc 14 and the plywood disc 20'. The magnetic flux path obviously includes the disc 14, the span of the bar-portion 12 that lies between the discs 12' and 24', and the soft steel disc 24'.

A centering means may be provided, if desired, on one or more of the. discs for insuring that the bar-portion 12 or 12 is along the axis of the tubular member A; and Fig. 4 indicates such a means applied to a disc. A plu rality of centering means of suitable construction may be attached, to the .disc. One such means 42 is shown in Fig, Laud comprises apair of radially spaced lugs 44 and 46 fixed to disc 14. The outer lug 46 is removable and has a slot that slidably receives bar 48 having a roller 50 journaled at its upper end. The bar 48 has an inner leg 52; and a compression spring 54 between the lug 44 and leg 52 tends to force the roller 50 outwardly against the tubular member A. A removable guide strap 56 helps to position bar 48 on the disc.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings; it is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An internal flaw indicator of a type described for testing the inner surfaces of tubes, comprising an elongated bar having a magnetic bar-portion at one end, and an extending portion extending from said bar-portion, said extending portion being adapted to be grasped by an operator whereby the indicator may be moved along the bore of a tube being tested, a plurality of discs in spaced relation on said bar-portion and extending radially therefrom, said bar-portion and a pair of said discs being magnetic, the last said discs having different diameters, a coil around said bar-portion and between said pair of discs, and means for adjusting the axial spacing between said discs.

2. An internal flow indicator of a type described for testing the inner surfaces of tubes, comprising an elongated bar having a magnetic bar-portion at one end, and an extending portion extending from said bar-portion, said extending portion being adapted to be grasped by an operator whereby the indicator may be moved along the bore of a tube being tested, a plurality of discs in spaced relation on said bar-portion and extending radially therefrom, said bar-portion and a pair of said discs being magnetic, said magnetic discs having different diameters,

' a third of said discs being nonmagnetic and between said pair of magnetic discs, a single coil only around said bar and located at said bar-portion, said coil being confined between said nonmagnetic disc and a first of said magnetic discs, the coil-ends of said coil comprising conductors extending along said bar.

3. An internal flaw indicator as defined in claim 2 but further characterized by the second of said magnetic discs being adjustable axially along said bar-portion.

4. An internal fiaw indicator of a type described for testing the inner surfaces of tubes, an elongated bar having a magnetic bar-portion at one end, and an extending portion extending from said bar-portion, said extending portion being adapted to be grasped by an operator whereby the indicator may be moved along the bore of a tube being tested, a plurality of discs in spaced rela tion on said bar-portion and extending radially therefrom,

said bar-portion and a pair of said discs being magnetic, said pair of magnetic discs having different diameters, a

coil around said bar-portion and between said pair of magnetic discs, means holding said coil in fixed position on said bar, and a first of said pair of discs being longitudinally adjustable on said bar-portion.

OTHER REFERENCES Article, Magnetic Particle Testing of. Commercial- Forgings, Forging Manufacturers Assoc., Inc., 1944. 

